Sodium hypochlorite is a chemical compound having the general chemical formula NaOCl. Sodium hypochlorite is used in a wide variety of applications, including uses as a disinfectant and/or a bleaching agent. In one application, sodium hypochlorite is marketed as a 3-6 weight percent solution for use as household bleach. Stronger solutions are marketed for use in the chlorination of water at water treatment plants, as a disinfectant in medical applications, and even stronger solutions are produced for chlorination of swimming pools. Sodium hypochlorite has been used for the disinfection of drinking water, at a concentration equivalent to about 1 liter of household bleach per 4000 liters of water. The exact amount required depends on the water chemistry, temperature, contact time, and presence or absence of sediment.
In some methods, sodium hypochlorite is prepared by absorbing chlorine gas into cold sodium hydroxide solution to induce the following reaction:2NaOH+Cl2⇄NaCl+NaOCl+H2OThe sodium hydroxide and chlorine reagents input into this process may be commercially produced by the chloralkali process. For use in this reaction, there is generally no need to isolate the reagents, thus, NaOCl may be prepared in an industrial setting by electrolyzing sodium chloride solution without any separation or barrier between the anode and the cathode. In this process, the reaction solution is generally maintained at a temperature below about 40° C. in order to prevent the formation of sodium chlorate. As a result, commercially-prepared sodium hypochlorite solutions generally contain amounts of sodium chloride as a primary byproduct.
Current methods for the production of sodium hypochlorite such as that described above are subject to several practical limitations. First, the current bi-polar or undivided cell technologies that are used to produce sodium hypochlorite cannot effectively handle the calcium and magnesium ions often contained in the salt solutions fed into the hypochlorite cells. As a result, without being limited to any one theory, it is believed that these ions begin to detrimentally effect the cells, in some cases causing formation of hydroxide precipitates and scale in process equipment, including within the cells themselves. This creates a requirement of timely and frequent cleaning of the cells to remove the scale and precipitate, thus adding servicing downtime as a practical limit to the production of sodium hypochlorite and adding further costs in the form of expenditures of resources and time in effecting such servicing. The cleaning processes used generally involve the use of hydrochloric acid which then generates additional chemical waste for disposal, thus incurring still further cost. If the cells are not cleaned in a timely fashion, however, the solid scale may penetrate between the electrodes and then potentially lead to short-circuiting of the electrodes, thus compromising the safety of the cell.
In addition to the above, it should be noted that the bi-polar or undivided cell process generates H2 gas mixed with Cl2 and other gases in combination. H2 gas is a highly valued energy resource, but it is impractical to recover it from the complex gas mixture output by currently-used hypochlorite cells, and it is, therefore, simply released, thereby wasting it.
One available alternative to the currently-used processes is an electrolytic membrane process using an organic membrane (such as Nafion). Such organic membrane-based processes often suffer from short membrane lifetime because calcium and magnesium ions originally present in the feed solution precipitate within the membranes, fouling them and reducing their efficiency and performance.
Thus, it would be an improvement in the art to provide a novel electrochemical process for generating sodium hypochlorite from a variety of sodium chloride input solutions, including seawater, that may be more efficient than currently-used processes, and which may generate less waste. It would be a further benefit in the art to provide a process for producing sodium hypochlorite that is resistant to fouling from the calcium and magnesium ions commonly found in such feed streams. It would be a further benefit in the art to provide a process for producing sodium hypochlorite that enables hydrogen gas to be practically recovered as a product.
Such processes and devices for conducting such processes are disclosed herein.